Chapter 078. Prevention and Early Detection of Cancer (Part 6) ppsx

Chapter 078. Prevention and Early
Detection of Cancer
(Part 6)

Cancer Screening
Screening is a means of detecting disease early in asymptomatic
individuals, with the goal of decreasing morbidity and mortality. While screening
can potentially save lives and has been shown to do so in cervical, colon, and
probably breast cancer, it is also subject to a number of biases that can suggest a
benefit when actually there is none. Biases can even mask net harm. Early
detection does not in itself confer benefit. To be of value, screening must detect
disease earlier, and treatment of earlier disease must yield a better outcome than
treatment at the onset of symptoms. Cause-specific mortality, rather than survival
after diagnosis, is the preferred endpoint (see below).
Because screening is done on asymptomatic, healthy persons, it should
offer substantial likelihood of benefit that outweighs harm. Screening tests and
their appropriate use should be carefully evaluated before their use is widely
encouraged in screening programs, as a matter of public policy.
Screening examinations, tests, or procedures are usually not diagnostic of
cancer but instead indicate that a cancer may be present. The diagnosis is then
made following a workup that includes a biopsy and pathologic confirmation.
A number of genes have been identified that predispose for a disease, and
many more will be identified in the near future. Testing for these genes can define
a high-risk population. The ability to predict the development of a particular
cancer may some day present therapeutic options as well as ethical dilemmas. It
may eventually allow for early intervention to prevent a cancer or limit its
severity. People at high risk may be ideal candidates for chemoprevention and
screening; however, efficacy of these interventions in the high-risk population
should be investigated. Currently, persons at high risk for a particular cancer can
engage in intensive screening. While this course is clinically prudent, it is not
known if it saves lives in these populations.

The Accuracy of Screening
A screening test's accuracy or ability to discriminate disease is described by
four indices: sensitivity, specificity, positive predictive value, and negative
predictive value (Table 78-2). Sensitivity, also called the true positive rate, is the
proportion of persons with the disease testing positive in the screen (i.e., the ability
of the test to detect disease when it is present). Specificity, or 1-false positive rate,
is the proportion of persons who do not have the disease and test negative in the
screening test (i.e., the ability of a test to correctly identify that the disease is not
present). The positive predictive value is the proportion of persons that test
positive who actually have the disease. Similarly, negative predictive value is the
proportion testing negative who do not have the disease. The sensitivity and
specificity of a test are relatively independent of the underlying prevalence (or
risk) of the disease in the population screened, but the predictive values depend
strongly on the prevalence of the disease.
Table 78-2 Assessment of the Value of a Diagnostic Test
a


Condition Present Condition Absent
Positive test a b
Negative test c d
a = true positive
b = false positive
c = false negative

d = true negative
Sensitivity
The proportion of persons with the condition who
test positive: a/(a + c)
Specificity

The proportion of persons without the condition
who test negative: d/(b + d)
Positive
predictive value (PPV)
The proportion of persons w
ith a positive test
who have the condition: a/(a + b)
Negative
predictive value
The proportion of persons with a negative test
who do not have the condition: d/(c + d)
Prevalence, sensitivity, and specificity determine PPV


a
For diseases of low prevalence, such as cancer, poor specificity has a
dramatic adverse effect on PPV such that only a small fraction of positive tests are
true positives.
Screening is most beneficial, efficient, and economical when the target
disease is common in the population being screened. To be valuable, the screening
test should have a high specificity; sensitivity need not be very high.